The present invention relates to speed change control for a vehicle with a continuously variable transmission (CVT), and especially to speed change control during operation of a traction control system.
When a traction control system (TCS) which suppresses tire slip by decreasing the driving force operates, a driving force changes, and a driving wheel speed fluctuates. Therefore, when a final target ratio is computed based on the driving wheel speed during traction control and speed ratio control is performed so that a real speed ratio approaches the final target ratio, the real speed ratio will fluctuate.
To prevent such a fluctuation, JP-A-H4-64764 published by the Japanese Patent Office in 1992 discloses a technique where the final target ratio is computed not based on driving wheel speed but based on an estimated vehicle speed, for example the driven wheel speed during operation of the TCS.
However, when the TCS operates, and speed ratio control is performed based on the driven wheel speed, although the above fluctuation is stopped, the driven wheel speed becomes smaller than the driving wheel speed as shown in FIG. 9, so it is difficult to perform an up-shift (speed change which makes the speed ratio small) compared with the case where driving wheel speed is used for speed ratio control.
When an up-shift becomes hard to perform, the engine rotation speed increases Moreover, if the driving wheels slip and the driving wheel speed increases, the rotation speed of the engine may increase too much.
It is therefore an object of this invention to prevent the engine rotation speed increasing excessively while suppressing speed ratio fluctuation during operation of a traction control system.
In order to achieve above object this invention provides a speed ratio control device for a vehicle comprising a continuously variable transmission and traction control system which controls a driving force. The device comprises a sensor which detects a vehicle speed, an actuator which changes the speed ratio of the transmission, and a microprocessor programmed to estimate a vehicle speed based on a running state, compute the target ratio of the transmission based on the sensor detected speed when the traction control system is not operating, compute the target ratio of the transmission based on the estimated vehicle speed, and limit the computed target ratio to a speed ratio upper limiting value computed based on the sensor detected vehicle speed, when the traction control system is operating, and control the actuator so that the speed ratio of the transmission approaches the target ratio.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.